6. What happens when :
a)
manganese dioxide is heated with conc.
hydrochloric
acid.​

Chlorine gas is produced

Explanation:

One of the methods of laboratory preparation of chlorine gas is to heat manganese dioxide with concentrated hydrochloric acid. The acid is oxidized by the manganese dioxide to liberate chlorine gas as follows;

MnO2(s) + 4HCl(aq) > MnCl2(aq) + 2H2O(l) + Cl2(g)

Hence, When manganese dioxide is heated with concentrated hydrochloric

acid, chlorine gas is liberated.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).